SPECIAL MODES OF VENTILATION:

1. High Frequency Ventilation - HFV: igh-frequency ventilation (HFV) has been one of the most studied ventilation techniques, apart from Positive Pressure Ventilation, over the past two decades. Despite its theoretical benefits, it has not received unanimous consensus and has not been widely used.

The most fundamental difference between high frequency ventilation (HFV) and intermittent positive pressure ventilation (IPPV) is that with HFV the tidal volume (Vt) required is approximately 1-3 ml/kg body weight, compared with 6-10 ml/kg with intermittent positive pressure ventilation (IPPV). The increase in ventilation rate to frequencies of 60 b.p.m. or more in HFV is obviously mandatory if even comparable minute volume ventilation is to result.

Three models are currently under investigation: High-frequency positive pressure ventilation (HFPPV), high-frequency jet ventilation (HFJV) and high-frequency oscillatory ventilation (HFOV). The first two are no longer used in intensive care therapy due to their poor results in trials compared to conventional mechanical ventilation. HFJV has found an important place in tracheo-bronchial surgery. HFOV is proving highly successful, mainly because adequate equipment capable of solving the problem of humidification of ventilated gases is now available (38, 39).

1.1 High Frequency Oscillatory Ventilation (HFOV): Tidal volume is delivered via normal sized tracheal tubes and both inspiration and expiration are active and of approximately equal power, such as would occur with an oscillating piston or loud speaker-based ventilator. Frequencies range from 2 Hz to more than 100 Hz (6000 c.p.m.). The ventilator is usually a reciprocating pump of the piston variety or a loudspeaker system driven by an electronic oscillator.

There are a number of mechanisms proposed to explain gas exchange in HFOV. Direct alveolar ventilation, asymmetric velocity profiles, Taylor dispersion, pendelluft, cardiogenic mixing, accelerated diffusion and acoustic resonance appear to participate in gas exchanges both individually and/or together.

Theoretical advantages:

• maintaining airways open
  
  
• smaller phasic volume and pressure change
  
  
• gas exchange at significantly lower airway pressures
  
  
• less involvement of cardiovascular system
  
  
• less depression of endogenous surfactant production.

HFOV is recommended in order to reduce lung barotrauma and consequent lung injury in non-homogeneous lung pathology, air leaks, Persistent Pulmonary Hypertension of Newborn (PPHN) and ventilation of premature babies.

Contraindications:

• pulmonary obstruction from fresh meconium aspiration
  
  
• bronchopulmonary dysplasia
  
  
• RSV bronchiolitis
  
  
• intracranial hemorrhage.

Complications:

• lung overinflation in obstructive lung diseases
  
  
• intracranial hemorrhages - reduction in heart rate attributed to increased vagal activity
  
  
• bronchopulmonary dysplasia due to inhomogeneous lung ventilation (Fig. 7)
  
  
• necrotizing tracheobronchitis, increased permeability of lung epithelium and insufficient humidification of tracheo-bronchial secretions.

Figure 7 - HFOV complications: Severe bronchopulmonary dysplasia complicated by total atelectasis of right lung.


While HFOV can maintain adequate gas exchange for prolonged periods in many situations, there is as yet no clearly defined clinical role for this mode of ventilation in pediatric patients. Despite the absence of any clearly defined clinical niche for HFOV, there seems little doubt that it will continue to be used extensively in bench testing and animal experimentation (40, 41).